Existing knowledge does not adequately explain many important features and mechanisms which are involved in the extraordinarily successful spread, establishment and maintenance of the zoonosis, murine typhys, in diverse habitats around the world, involving a variety of small mammals and ectoparasites. This study will examine, under controlled conditions in the laboratory, using modern quantitative methods, selected components and interactions of the rickettsia-small mammal-ectoparasite complex which have been identified as potentially critical by field and preliminary laboratory observations. The fundamental parameters controlling the dynamics and stability in the rickettsia-small mammal-ectoparasite system have not been quantitatively analyzed. A thorough understanding of these parameters is essential for proper management of outbreaks of murine typhus. Our objectives are four fold. 1) To investigate in detail the Rickettsia mooseri (R. tphi)-ectoparasite interactions using two species of fleas xenopsylla cheopis and Leptopsylla segnis), with respect to rickettsial growth and cytopathology and the longevity of the infected arthropods. 2) study in detail the mechanisms of intermurid transmission and of R. mooseri by ectoparasite, including dust-borne rickettsia in their dried feces, effluvia, etc. 3) Search for special "homing" and "retrieval" mechanisms of r. mooseri in infected non-immune, passively immunized and actively immunized rats, whereby acquisition of infection by the vector is facilitated. 4) Define the influence of humoral and cellular immune factors in the rat on the efficiency of vectors.